1. Field of the Invention
The present invention relates to a driving apparatus for driving an electric motor according to a given control target.
2. Related Art Statement.
A sewing machine performs a sewing operation by converting rotary motion of an electric motor into vertical motion of a needle bar to which a sewing needle is secured, vertical motion of an upper(needle)-thread take-up lever with different operation timing than that of the vertical motion of the needle bar, rotary rotation of a shuttle in which a lower thread is accommodated, X--Y feeding of a work sheet such as a cloth or leather relative to the needle (or needle bar), etc. The electric motor employed in the sewing machine is, e.g., a universal motor. FIG. 4 shows a conventional driving apparatus for driving a universal motor 4. A driving apparatus employed in a sewing machine is an example of the motor driving apparatus to which the present invention relates.
As shown in FIG. 4, a universal motor (M) 4 is connected to a commercially available AC (alternating current) power supply 1 via a noise-removing circuit 2 and a solid-state relay (SSR) 3. Each of the rotor and stator of the universal motor 4 is formed of a coil. The AC source voltage of the AC power supply 1 is directly applied to the universal motor 4, so that the rotor of the motor 4 rotates in one direction irrespective of the changing of flow directions of the electric current flowing through the motor 4. FIG. 5(A) shows a waveform of the AC source voltage of the AC power supply 1.
A central processing unit (CPU) 5 is connected to the solid-state relay 3. A target-speed volume 6 and a rotary encoder 7 are connected to the CPU 5. The volume 6 is manually operable by an operator or user to input or preset a desired target rotation speed of the universal motor 4. The encoder 7 detects an actual rotation speed of the motor 4. The CPU 5 compares the actual motor speed detected by the encoder 7, with the target motor speed preset through the volume 6, and produces a control command signal based on the comparison result. The CPU 5 utilizes the control command signal for determining a timing to supply an ON signal to the solid-state relay 3. FIG. 5(B) shows the ON signals which the CPU 5 supplies to the relay 3. Upon reception of each ON signal, the relay 3 permits a portion of the voltage waveform of the AC supply 1 shown in FIG. 5(A) to pass therethrough to the universal motor 4, so as to rotate the motor 4. This "portion" of the AC voltage waveform starts with a phase angle upon reception of each ON signal and ends with the following zero crossing.
The CPU 5 modifies the control command signal and changes the timing of supplying of an ON signal so that the actual motor speed detected by the encoder 7 gradually approaches the target motor speed preset via the volume 6. For example, in the case where the actual motor speed measured by the encoder 7 is lower than the target motor speed input through the volume 6, the CPU 5 shifts the timing of supplying of an ON signal to the relay 3, leftward as seen in FIG. 5(B), that is, generates an ON signal at a shorter interval. Consequently, a greater portion of the AC voltage waveform is supplied to the universal motor 4, so that the rotation speed of the motor 4 is accelerated.
The voltage waveform of the AC supply 1 shown in FIG. 5(A) oscillates at 50 Hz or 60 Hz, therefore the universal motor 4 rotates at a frequency of 100 Hz or 120 Hz as shown in FIG. 5(B) or FIG. 5(C). FIG. 5(D) shows a waveform of an electric current flowing through the motor 4. Meanwhile, the output torque of a DC (direct current) motor changes in proportion to the magnitude of electric current flowing through the DC motor. In particular, the output torque of a universal motor 4 changes in proportion to the square of electric current flowing therethrough. FIG. 5(E) shows the change of output torque, .GAMMA., of the universal motor 4 with respect to time, t.
However, the conventional motor driving apparatus arranged as described above suffers from the problems that the universal motor 4 intermittently generates output torque peaks .GAMMA. as shown in FIG. 5(E) and that the respective maximum values of the intermittent output torque peaks .GAMMA. are not constant or uniform. Consequently the motor 4 produces vibration having a frequency of 100 Hz or 120 Hz. This vibration of the motor 4 causes discomfort vibration of the framework of the sewing machine which in turn is transmitted to the operator or user. Additionally, since the 100 Hz or 120 Hz vibration falls within the human audible sound range, the operator or user cannot avoid hearing discomfort low-tone noise resulting from that vibration. A test shows that a home-use or domestic portable sewing machine produces noise as high as 57 dB when a universal motor employed therein is rotated at 110 rpm (rotations per minute).
In this background, it may be considered that the above problems may possibly be solved by the following methods: the first method is to employ a vibration absorber such as a rubber member and the second method is to drive a universal motor with a direct current supplied from an exclusive DC power supply. However, the first method does not fundamentally eliminate the cause of the vibration, therefore cannot sufficiently reduce either the vibration or the noise. In addition, since the vibration absorber is expensive, the production cost of the sewing machine is much increased. In the second method, a considerably large DC power supply is needed for driving a universal motor. This also increases the production cost of the driving apparatus. In addition, it may be required that a heavy and bulky element such as a transformer be incorporated into the driving apparatus. In either case, the sewing machine cannot satisfy commercial requirements on either size or price.
In the conventional sewing machine of FIG. 4 having the universal motor 4 and the control apparatus for the motor 4, the rotary motion of the motor 4 is utilized for responding to not only constant load changing such as vertical reciprocation of a needle bar, but also abrupt load changing such as taking up of a needle thread by the needle-thread take-up lever. However, it is difficult to effectively follow each load changing at a control timing or frequency of 100 Hz or 120 Hz. Thus, the conventional driving apparatus suffers from the problems of delayed response to the load changing and resultant ineffective control of the motor rotation speed.
FIG. 6 shows another conventional driving apparatus for a DC motor 8. In this driving apparatus, a rectifying element 9 is provided between a noise-removing circuit 2 and the DC motor 8. A CPU 5 controls the phase of full-wave rectified output of the rectifier 9, thereby driving the DC motor 9. However, the second driving apparatus suffers from the same problems as those with the above described, first driving apparatus.